Saturday, April 21, 2012

Learning Objectives

Equipment layout is based on certain considerations. The final plant design is executed on the this drawing and it should be seen that plant should be safe and economical. This equipment layout is very important because this is used for the execution of plant.

The piping engineer has to understand the concept of equipment layout and consideration. While preparing layout, the piping engineer should design a steady process, non-hazardous utility and facility.

Introduction

In general the various process plants we come across are petroleum refineries, petrochemical complexes, fertiliser plants, and chemical and pharmaceutical plants which need special attention due to the nature of materials and products handled.

Before starting the development of the plant layout, the following information is required.

Ideally, before site selection, a preliminary layout should be made. The data collected during the site selection stage shall be given due consideration at the time of the equipment layout. A few points to be considered are:

i. Site location, map and area topography, geological and meteorological data

Major Consideration in Plant Layout

The most economical plant layout is that in which the spacing of the main equipment is such that it minimises interconnecting pipe work and structural steel work. Normally equipment should be laid out in sequence to suit the process flow but exception to this arises from the desirability to group certain tanks, pumps or perhaps to isolate hazardous operations according to statutory rules and regulations. The use of the single stream or multiple stream flow pattern will affect the layout.

The major variables affecting the final layout are interconnected pipe sizes, insulation thickness, steel work spacing, matters of operational convenience, safety, ease of erection and maintenance which calls for expiries of critical judgment on the experience and the study of existing and know limitation.

Economic

Basically economic consideration means installing the unit in the smallest possible space, consistent with the operability ease of piping material, structural sheets and concrete with the proper layout considering further economies which can be achieved in way of pumping and utilities.

Safety

Where toxic or hazardous materials are handled, layout may be needed to isolate a section of the plan equipment, which could be a possible source of hazard. It should be grouped together and where possible located separately from other areas of the plant. i.e. furnaces, flare stacks or other equipment containing naked flares. Mechanical equipment handling flammable or volatile liquids which could easily leak or spill out of the equipment thus causing flammable conditions. However such consideration should not override considerations of cost. For instance, the process heater must necessarily be located close to other equipment to conserve expensive alloy piping. Furnaces using gas as a feedstock do not normally constitute hazards. Due consideration shall be given for fire hazardous areas and isolated by providing firewall, fire door, etc. The BOCA (Building official and code administration) published by the National Building Code shall be followed.

The area equipment area handling acids or other toxic material, which cause damage or endanger personnel by their spillage should in general be grouped together and isolated.

To isolate hazardous areas it may be necessary to build walls with self-closing doors. Definitions of hazardous areas are given in NFPA, NEC, and the Petroleum Act OISD or local standards.

Proccess

Process considerations may require some items be elevated to provide gravity flow of materials to accommodate pump suction requirement for NPSH. The other process consideration could be the limitation of pressure or temperature drop in the transfer lines deciding the proximity of the furnace, reactors and columns.

Operational

Thought should be given to the location of equipment requiring frequent attendance by operation personnel and the relative position of the control room to obtain the shortest and most direct route for operator. When on routine operation however the control room should be placed so that they are easily accessible and the indicators are easily readable. Generally a batch or semi batch process needs more attention by the operator and therefore more consideration has to be paid to the ergonomics of the layout.

Maintenance

The need to remove for servicing, retuning or replacements, the heavy, servicing equipment. The indivisible plant will dictate their location when access for cranes is called for regular or rotating and other machinery calling for dismantling. This often makes their grouping within the machine house desirable. The position of items needing replacement of internal spent catalyst etc. or frequent internal cleaning has to be carefully considered.

Constructional

The plot should be so designed that adequate access is available to lift the large items of equipment or columns into place. Such equipment is positioned close to the boundary limits so that erection must take place from outside these limits. A careful check must be undertaken to ascertain whether space will be available at the time of erection for positioning the crane or lifting the delivery equipment which is known, may well arrive fairly late in the construction programme and therefore have to be fitted into place after most of the surrounding equipment has already been installed. It is important that the insulation requirement be considered during the layout of the plant.

Appearance: Aesthetic

An attractively laid out plant with the equipment in rows also economically laid out gives an aesthetic appearance. The building structure and groups of equipment should form a neat, symmetrical balanced layout consistent with keeping the pipe run to a minimum and allowing proper access for maintenance. The tower and large vertical vessels should be arranged in rows with a common centreline. They should be of similar size but if the diameter varies, due note must be taken of the building line manholes on the adjacent tower should be at a similar elevation and orientation to streams. These as far as possible should be made identical. Such arrangement for parallel streams or similar groups of process equipment require much the design work and also for construction and subsequently operation but help in reducing the amount of standby equipment.

Future Expansion

Thought should be given to the likely expansion of both equipment and pipe work, so that additions can be erected and tested with the minimum interference to plant operations. On main pipe runs, it is desirable to leave 30% space. At least 30 m distance from flame proof plant area is needed for safe welding where no special precaution are needed.

Buildings

Plant buildings should be kept to a minimum on the basis that most of equipment including pumps, heat exchanger boilers, cyclones, etc. may be safely installed in the open. The philosophy should be that the plant is supported on an open steel structure unless there is good reason not to do so. Thus it is to be installed in the open with centralised control facilities housed in buildings.

Factors which determine the selection of enclosed buildings are;

b) Climate- Extreme climate conditions may determine that the plant is to be kept in a building.

c) Type of equipment - Expensive equipment and complex machinery should have some degree of weatherproofing.

d) Nature of process - Plants handling dust explosive and combustible solid materials require a building .To prevent contamination, food, pharmaceutical and bio-chemical plants require to be in a building.

e) Proximity of hazards- To prevent the possible spread of fire and explosion, a building may be desirable.

f) If enclosed buildings are planned, then consideration must be given to the following factors:

i) Fire protection/Explosion Protection approval by the local fire authority must be obtained on fire fighting equipment and layout.

ii) Illumination.

Natural illumination may be obtained by the use of patent glazing windows or translucent sheets in the sidewalls or the roof. Artificial lighting must be arranged to give adequate illumination where physical and chemical hazard exists-

iii) Fireproofing - Determines the plant arrangement and switch room.

iv) Ventilation - Air intake should be positioned in such a way that it should avoid the risk of drawing toxic or hazardous fumes. Exhaust may be required for treatment of filter washer, separation.

v) Heating

vi) Access-Proper access for maintenance and operation purposes to be considered.

vii) Roofing.

Techniques for Layout

There is no single technique leading to the best arrangement in any problem. Layout may be considered at several stages from the selection of the site and arrangement of the plant on it through block layout. The three basic principles of layout planning are:

1) Plan the whole, then the detail, Individual aspects must be subservient to the whole and sub optimisation avoided.

2) Plan the ideal, then the practical. The ideal layout is free from restrictions and gives a datum.

3) Plan more than one layout. It is seldom that a single layout is "BEST" for each criterion. Planning more than one permits comparison and leads to greater confidence in making the final selection.

4) Sequence of activities in layout. A simplified network showing the integration of the plant layout and design activities should be given.

Details of Techniques for Layout

1) Initial development of the layout.

• Layout is by process flow, in the sequence of operation and materials moved.

• Critical Examination - question

Where is the plant item placed?

• The steam ejector at elevation.

Why it is placed there?

• Convenient for barometric leg.

Where else could it be?

• Near the ground level with the condenser and the vacuum pump. Correlation and compatibility - what is impossible or inadmissible.

2) Techniques of correlation and compatibility. The correlation chart is a diagrammatic method of determining the effect of constraints.

3) Proximity and sequencing - Travel chart. The simplest manual method is probably one of trial and error in which items are situated near one another as suggested by significant costs and the arrangement modified with reference to the travel chart.

Layout Analogues

2) Block models -These models are chiefly used to develop the plot plan, floor plan, elevation, major equipment and major pipe racks in the correct co-relation with each other.

3) Drawings- for the development and evaluation of the layout it co- relates the basic project information on the equipment and site condition and provides source of data for the final layout.

4) Piping models- for accurate detailed layout of the process piping utility and control facilities.

Computers

If there are six alternate positions for each of one of the six blocks then there are 720 feasible combinations of these six blocks. In general, for a computerised plant layout, the judgment of the engineer has to be combined with the calculation power of the computer. Usage of the computer is on the evaluation of systematic techniques. Small problems can be handled manually with various co-relation and compatibility methods.

Concept of the Plant Layout

A process plant which consists of the various different sections mentioned earlier should be arranged so as to follow the general route of the raw material, to process, to intermediate/finished product stage to dispatch.

The entire plot area will be divided into blocks, the size of the blocks depend on the facilities to be accommodated.

While locating the blocks and further sub-blocks within them, the following points runs should also be minimum.

1) The layout planning shall be in the sequential order of the process flow so that the piping run is also minimum.

2) The block shall be arranged considering the prevalent wind direction such that flammable gases should not be carried by the wind onto a source of ignition.

3) The process unit block shall be centrally located with a straight approach from the main gate.

4) Storage tanks shall be grouped according to product classification. In undulating areas, storage tanks shall be located at lower elevations.

5) Utility blocks shall be located adjacent to the unit block.

6) The main power receiving station shall be close to the boundary line so that the minimum overhead power lines pass through the plant.

7) The electric sub- stations in the plant shall be located in such a way that cabling to major load centres are minimum.

8) The location of power plants shall be near the process/utility block and close to the main receiving station.

9) Flares shall be located upwind of the process units so that the inflammable gas from the plant are not carried towards the flares.

10) Truck loading facilities shall be located close to the product movement gate. Rail loading facilities are arranged generally at the periphery of the plant.

11) The effluent plant shall be located away from the other units on the down elevation. The preferred location is at the lower elevation than the other plant units in order to facilitate gravity flow.

12) The fire station and firewater pump house shall be at a safe place away from hazardous areas. The fire station shall be near the main gate with a straight approach to the process units and other critical areas.

13) Two adjacent process unit locations shall be decided based on the annual shut down philosophy for the maintenance of the units. If the shut downs are be at different times, it is preferable to increase the distance between the two units so that the risk for the operating units due to heat jobs in the units under shut down can be minimised. Also this will facilitate easy maintenance.

14) Process Units shall be located on the high ground.

15) The adjacent neighbourhood installations, if any, shall be taken into consideration before fixing the plant layout. The limitations imposed by the neighbourhood facilities cannot be ignored, e.g. an ignition source.

16) Flares, furnaces/heater, dusty operations and cooling towers shall be oriented depending on the prevailing wind direction. The first two shall be located upwind of the Process units and the last two on the downwind directions of the process units.

17) Due consideration to the construction and erection of the plant shall be given while deciding the plant layout, especially tall towers, reactors, furnaces, etc. shall not be in congested areas and sufficient open spaces shall be provided for erection at any stage.

18) Equipment requiring frequent maintenance shall have easy accessibility. So also equipment which need removal of parts and also for free access for hoisting equipment.

19) Provisions for future expansion shall be considered. It is preferable to have similar type of facilities with adjacent space for expansion adjacent. Care shall be taken to install the expansion facilities without any disturbance to the existing facility.

20) Roads in between for the purpose of access and safety should separate the blocks developed. Fire fighting facilities are to be provided on these roads.

There shall always be an alternative approach for fire fighting and maintenance in case one route is blocked. The layout shall be so designed that truck traffic inside the plant is minimized.

21) Boiler house, air compressors, fuel oil facilities shall be grouped into one block adjoining the process unit blocks, so that spreading of various utilities can be avoided and also facilitate easy operation.

22) The layout shall be designed so as to minimise the capital cost for earthwork and pavement, special foundations and the extent of a vacant plot as wasteland.

For the major distances to be considered during plant layout OISD (Oil Industries Safety Directorate) Standards shall be referred to.

The oil industry safety directorate (OISD) gives some guidelines for plant layout but does not overrule the existing statutory requirements.

The environment/security requirements now demand that a Green Belt area around the plant boundary shall be provided. This is a belt of width 50m to 100m around the plant boundary where no construction will be done.

The Present Fact

Alternatives

Selection for Development

What is achieved?

why ?

what else
could be achieved?

what should be achieved?

How it is achieved?

Why that way?

How else could it be achieved?

How should it be achieved?

When it is achieved?

Why then?

When else could it be achieved?

When should it be achieved?

Where is it achieved?

Why there?

Where else could it be achieved?

Where should it be achieved?

Who achieves it?

Why that persion?

Who else could achieve it?

Who should achieve it?

(a) Critical examination sheet

Stages in the development of the layout

Useful Technique

1)The layout begins with the collection and assessment of the flow sheet ( which must show material of construction, size of pipe work and suggested elevation ), equipment data sheets, result of site survey (incorporating relevant data on site boundaries, topography, soil structure, weather conditions, utility terminals and site access points).

Critical examination is used to question the elevation layout assumptions made in the flow sheet. This method enables the objectives and constraints to be defined. Techniques of correlation and compatibility are used to sort and list the various alternatives.

2) The cost of each elevation alternation is examined for differences only e.g. in the number of plant items needed to achieve the objectives in the material transfer costs such as piping elevation, power consumption.

Simple elevation drawings are prepared for the alter natives showing heights and relative positions only of plant items only, No structure or floor levels are added yet as these may depend on plan layout.

3)Plant items and buildings are laid out in the plan ensuring that all layout constrains (e.g. relating to operation, maintenance, safety costing environment and expansion ) are applied A similar costing made of each plan layout is made

Critical examination is used to question the plan layout assumptions made in the defined Once the constraints are defined Techniques of correlation and compatibility are again used to sort and list the various alternatives. Layout analogous such as cut outs help in the visualization of the layout and in the examination of plot size and external constraints.

4) The selected plan and elevation layout are now combine to determine the possible positions of supporting and access structures, and to study the civil requirements. These may force the relaxation of earlier constraints.

The layout alternative is presented by block models. This stage will help people to visualise the non-quantifiable and safety aspects.

5) Final alternatires are again costed for differences and a selection is made.

The final layout is drawn (options may still be left open for detailed study)

6) Preparation of piping models now commences. These are further used to optimize pipe work .A check should be made that all constraints are obeyed particularly those for operation.

Piping models are now prepared.

(b) Typical stages in plant layout

Summing Up

Understanding of equipment layout is very important and is based on certain considerations and guidelines. The concept behind the preparation and application of the layout are very important. The plant is executable based on this important layout.